In order to solve depletion of fossil energy and earth-environmental problems caused by using fossil energy, research into alternative clean energy sources such as solar energy, wind energy, and hydro energy that are recyclable and clean has been actively conducted.
Among them, an interest in a solar cell directly converting solar lights into electric energy has significantly increased. Here, the solar cell means a cell generating current-voltage using a photovoltaic effect that the cell absorbs light energy from the solar lights to generate electrons and holes.
Currently, an n-p diode type single-crystalline silicon (Si) based solar cell having photovoltaic conversion efficiency higher than 20% may be manufactured and actually used in solar power generation, and there is a solar cell using a compound semiconductor such as gallium arsenide (GaAs) having conversion efficiency higher than that of the n-p diode type single-crystalline silicon (Si) based solar cell. However, since these inorganic semiconductor based solar cells require a very highly purified material for high efficiency, a large amount of energy is consumed in purifying a raw material, and expensive processing equipment is required during a single crystallization process or a thinning process using the raw material, such that there is a limitation in lowering manufacturing cost of the solar cell, thereby blocking large-scale use of the solar cell.
Therefore, in order to manufacture the solar cell at low cost, cost of a core material used in the solar cell or the manufacturing process of the solar cell should be greatly reduced, and research into a dye-sensitized solar cell (DSSC) and an organic photovoltaic that may be manufactured using an inexpensive material and process has been actively conducted as an alternative to the inorganic semiconductor based solar cell.
The dye-sensitized solar cell (DSSC) was initially developed by Michael Gratzel in 1991, a professor at EPFL in Switzerland and was reported in Nature.
An early dye-sensitized solar cell had a simple structure in which a dye absorbing light was adsorbed on porous photoanodes on a transparent electrode film through which light and electricity flow, another conductive glass substrate was positioned on the film, and a liquid electrolyte was filled therebetween.
An operation principle of the dye-sensitized solar cell is as follows. When dye molecules chemically adsorbed on surfaces of the porous photoanodes absorb solar light, the dye molecules generate electron-hole pairs, and electrons are injected into a conduction band of semiconducting oxides used as the porous photoanodes to be transported to the transparent conductive film, thereby generating current. The holes remaining in the dye molecules configure a complete solar cell circuit in a form in which the holes are transported to photocathodes by hole conduction due to an oxidation-reduction reaction of a liquid or solid electrolyte or a hole-conductive polymer, thereby performing external work.
Meanwhile, the organic photovoltaic (OPV) is configured of organic materials having electron donor (D, or often called a hole acceptor) characteristics and electron acceptor (A) characteristics. When the solar cell made of organic molecules absorbs light, electrons and holes are formed, which are called exciton. The exciton moves to a D-A interface, such that an electric charge is separated, an electron moves to the electron acceptor, and the hole moves to the electron donor, thereby generating photo current.
Since a distance at which the exciton generated in the electron donor may move is about 10 nm, which is significantly short, photo active organic materials may not be thickly laminated, such that optical absorbance was low and the efficiency was low. However, recently, due to introduction of a so-called bulk heterojunction (BHJ) concept of increasing a surface area at an interface and development of an electron donor organic material having a small band gap to easily absorb solar lights in a wide range, the efficiency was significantly increased, such that an organic photovoltaic having efficiency higher than about 10% has been reported.
In the organic photovoltaic, a manufacturing process of a device is simple as compared to the existing solar cell due to high formability of the organic material, diversity thereof, and low cost thereof, such that the organic photovoltaic may be manufactured at low cost, as compared to the existing solar cell. However, the organic photovoltaic has a problem in that a structure of the BHJ is degraded by moisture in air or oxygen, which rapidly decreases the efficiency of the solar cell, that is, a problem in the stability of the solar cell. When a technology of completely sealing the solar cell is introduced in order to solve this problem, the stability may be increased, but the cost may also be increased.
As a method of solving problems of the DSSC by the liquid electrolyte, an all-solid state DSSC using Spiro-OMeTAD[2,2′,7,7′-tetrkis (N,N-di-p-methoxyphenylamine)-9,9′-spirobi fluorine], which is a solid state hole conductive organic material, rather than the liquid electrolyte to thereby have efficiency of 0.74% was reported in Nature in 1998 by Michael Gratzel, a chemistry professor at EPFL in Switzerland, who is an inventor of the DSSC.
Therefore, research into a technology of applying the spiro-OMeTAD, which is a hole conductive material, to a perovskite solar cell to achieve high efficiency was conducted (J. Burschka, N. Pellet, S. J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin and M. Gratzel, Nature, 2013, 499, 316-319).
However, a solar cell having high efficiency capable of being commercialized has been still required.